Conventionally, a silicon wafer is produced by slicing a silicon single crystal ingot. As a method for growing a silicon single crystal ingot, there is CZ (Czochralski) method, FZ (Floating Zone) method, or the like, FZ method is better than CZ method in that it is possible to obtain a single crystal of high purity.
FIG. 3 is an explanatory view of growing a silicon single crystal ingot according to FZ method. First, a polycrystalline silicon raw material rod 1 of which a tip end is conically processed is held vertically. On a vicinity of a lower end thereof, there is almost coaxially disposed a tip end of a seed crystal 2, for example, having a square rod (or a cylindrical rod), 5 mm on a side (or a diameter), consisting of a silicon single crystal. A lower end of the polycrystalline silicon raw material rod 1 is heated to melt by a radio-frequency induction heating coil 3. The seed crystal 2 is fused to the melted part of the lower end of the polycrystalline silicon raw material rod 1, and then furthermore, an upper end of the seed crystal 2 is gradually melted. After reaching thermal balance, the single crystal growth starts with the seed crystal 2. In this case, an orientation of the growing crystal is the same as an orientation of the seed crystal 2.
Here, by thermal shock caused when the seed crystal 2 is fused to the silicon-melted part being at a high temperature on the lower end of the polycrystalline silicon raw material rod 1, slip dislocations are introduced at high-density in a part for growing a crystal following a seed crystal 2. Because the introduced slip dislocations cause polycrystallization of a crystal to be grown, for making the slip dislocations disappear, there is performed a so-called necking that a neck portion 4 is formed by narrowing down the single crystal-growing part so that a diameter thereof becomes, for example, 2-3 mm. By the necking, the dislocations are led outward to the direction of the single crystal growth and made to disappear at the neck portion 4. The crystal-growing part is made to be dislocation-free, for example, by leading the dislocations outward sufficiently by forming the neck portion 4 with a length of 20 mm or more, and thereafter increasing of a diameter thereof is initiated.
Next, from the initiation of the increasing of a diameter of the crystal-growing part to reaching a desired diameter, a single crystal is grown at an inverted conical form and thereby a cone part 5 is formed. After reaching the desired diameter, the single crystal is grown with controlling a crystal growth rate or a temperature so as to become constant in the desired diameter, and thereby so-called a straight body is formed. Sequentially, a silicon single crystal ingot 6 is grown along with forming a floating zone 7 by moving the heating coil 3 upward in the axial direction relatively to the polycrystalline silicon raw material rod 1. When the floating zone 7 reaches an end of the effective length of the polycrystalline silicon raw material rod 1, the silicon single crystal ingot 6 is disconnected. The silicon single crystal ingot 6 grown as described above is made to be dislocation-free by the necking as described above. The necking is widely known as Dash Necking method.
FIG. 4 is an X-ray observation view showing an aspect of dislocation-free growth by Dash Necking method. This is an X-ray observation view of a crystal cross-section of the seed crystal 2′ to the conical part 5′ when dislocation-free growth is performed by Dash Necking method in the growth of the silicon single crystal ingot whose crystal axis is a crystal orientation <111> by FZ method. Parts appearing in the form of a black line in the neck portion 4′ are slip dislocations. According to the observation view expanding the neck portion 4′, there can be observed an aspect that slip dislocations propagate outward to the axis for crystal growth and disappear.
Conventionally, a silicon single crystal ingot grown according to CZ method or FZ method as described above has mainly had an crystal orientation <100> or <111>. This is because silicon wafers having an orientation <100> or <111> have been mainly used for fabrication of semiconductor devices because of advantage in physical property or processes of growing a crystal or producing semiconductor devices.
FIG. 2 is a flow chart showing an example of a process for producing a silicon wafer according to a conventional FZ method. First, for example, there is prepared a seed crystal having its crystal axis with the just angle of a crystal orientation <111>. By using the seed crystal, there is grown a silicon single crystal ingot having its crystal axis with the just angle of a crystal orientation <111> by FZ method according to the above-described method. When the ingot is grown, dislocation-free growth is performed by Dash Necking method. Next, the grown ingot is cut to be cylindrical blocks and ground cylindrically and thereafter subjected to Orientation Flat process or Orientation Notch process. The obtained ingot is sliced in the direction perpendicular to the crystal axis of the ingot by using a slicing means such as an inner diameter slicer, a wire saw, or the like. Thereafter, a chamfering process, a lapping process, an etching process, and a polishing process are sequentially performed, and there is produced a silicon wafer having a desired crystal orientation <111>.
A silicon wafer having a crystal orientation <100> or <111> has been mainly used for fabricating semiconductor devices as described above. However, it has been noticed in the recent years that, because carrier mobility of a semiconductor device is greatly dependent on a crystal orientation of the wafer, the carrier mobility can be larger by using a silicon wafer having a crystal orientation <110> with respect to requests for speeding up of an working speed of a semiconductor device. For example, speeding up of a working speed of a semiconductor such as a switching rate can be expected. Therefore, the demand of the wafer is increasing.
However, it is known that there is a problem for growing a silicon single crystal ingot having its crystal axis with a crystal orientation of just <110> by CZ method or FZ method. Propagation of slip dislocations introduced in the crystal-growing part by thermal shock is determined by the crystal system thereof. However, because a crystal lattice structure of the silicon single crystal is a diamond structure, a plane of slipping of dislocations (a dislocation plane) is {111}, which is a plane of closest packing of atoms, and the slipping direction, namely the propagating direction of dislocations, is an orientation <110>, which is the direction of the shortest lattice vector.
In a silicon single crystal ingot having its crystal axis with an crystal orientation <100> or <111>, the dislocation plane has a degree of 54.74° or 70.53° respectively with respect to the crystal plane which is a sliced surface when the ingot is sliced in the direction perpendicular to the crystal axis. In the case that a dislocation plane and the crystal plane has such an angle, even if a crystal orientation is made to be the same as the direction of the single crystal growth when the single crystal is grown, the generated dislocations in the seed crystal can be led outward in the direction of the single crystal growth and made to disappear sufficiently by Dash Necking method as described above. However, when it is attempted to grow a single crystal having its crystal axis with a crystal orientation of just <110>, the direction of the single crystal growth corresponds with the direction of the propagation of slip dislocations. Therefore, even if Dash Necking method is performed, dislocations are not sufficiently led outward in the neck portion and thereby not made to disappear sufficiently and remain in the grown ingot and cause polycrystallization. Even if Dash Necking method is applied when a silicon single crystal ingot having its crystal axis with a crystal orientation <110> is grown as described above, it has been extremely difficult to obtain a dislocation-free silicon single crystal ingot successfully.
Therefore, in Japanese Patent Laid-open (Kokai) Publication No. 9-165298, Lawrence D. DYER “Dislocation-Free Czochralski Growth Of <110> Silicon Crystals”, Journal of Crystal Growth vol. 47 (1979) pp. 533-540, and M. R. L. N. Murthy and J. J Aubert “Growth Of Dislocation-Free Silicon Crystal In The <110> Direction For Use As Neutron Monochromators”, Journal of Crystal Growth vol. 52 (1981) pp. 391-395, and such, there was disclosed a technology of growing a dislocation-free silicon single crystal wafer having a crystal orientation <110> by using CZ method. For example, in the above references of L. D. DYER and M. R. L. N. Murthy, there is disclosed a method that by forming only plural sets each having a neck portion and a diameter-increasing part for during the necking when a silicon single crystal is grown according to CZ method, there is produced a dislocation-free silicon single crystal having a crystal orientation <110>. Moreover, in the Japanese Patent Laid-open (Kokai) Publication No. 9-165298, there is disclosed a method that by narrowing down the diameter to be slim, less than 2.0 mm, with applying magnetic field during the necking, dislocations are made to disappear when a silicon single crystal is grown by CZ method.
With regard to CZ method, it has been conventionally attempted to grow a silicon single crystal having its crystal axis with a crystal orientation <110> according to such a method as described above. On the other hand, with regard to FZ method, an effective method has not been established yet. Therefore, in the case that it is attempted to obtain a silicon wafer having a crystal orientation <110> by FZ method, it is difficult to grow a dislocation-free silicon single crystal having a crystal orientation <110> by a conventionally method, and if a crystal is grown, polycrystallization is frequently caused and success probability of dislocation-free growth is low, and therefore the crystal has been remarkably low in both sides of yield and productivity.